The long-term goal of this project is to understand the role of endothelin-A receptor (Ednra) signaling in cephalic neural crest cells (NCCs) during craniofacial morphogenesis. NCCs within the pharyngeal arches are patterned by a wide array of transcription factors and signaling molecules. Expression of the genes encoding these proteins are generally confined to specific "domains" within the arches, though very little is known about how these domains are established. Endothelin-1 (Ednl)-induced signaling of Ednra is crucial for NCC patterning during lower jaw development, as Edn1-/- and Ednra-/- embryos die at birth from severe craniofacial birth defect that include homeotic transformation of first mandibular first arch-derived structures into more maxillary first arch-like structures, indicating a loss of cell identity. These changes are preceded by disruption of normal mandibular arch gene expression. Our preliminary data indicates that Ednra signaling may be required for NCCs to populate the distal arch. We hypothesize that Edn1/Ednra signaling creates a "development domain" within the mandibular arch by two methods: 1) guiding NCCs into specific mandibular arch compartments by establishing an Edn1 gradient within the arch and 2) inducing mandibular-specific gene expression while inhibiting maxillary-specific gene expression, with the induced gene expression dependent on the source of Edn1 within the arch. We will address these hypotheses using a variety of approaches. In Aim 1, we will use an inducible gene expression system in the mouse to target Edn1 expression to NCCs, allowing us to disrupt any Edn1 gradient in the pharyngeal arches. In Aim 2, we will use Cre/loxP technology in the mouse to individually inactivate the Edn1 gene in its source tissues within the arches. In Aim 3, we will use the Dlx1/Dlx2 and Ednra mutant strains and a novel inducible Dlx2 transgenice mouse strain to examine how Edn1/Ednra signaling establishes a developmental program in the mandibular arch. Together, these approaches will hopefully elucidate the functional significance of endothelin signaling during facial development, further our understanding of the molecular basis of mandibular arch patterning and help explain the molecular basis behind human birth defects that affect the face.